Vacuum easels are well known in the photographic arts for releasably holding a sheet of paper, photographic film or other flat material. Conventional easels are relatively thick and provide a relatively uniform vacuum over the entire surface of the easel. The thickness of the easel limits the size of prints that can be made with an enlarger disposed over the easel because it reduces the maximum separation distance between the enlarger and a sheet of paper held by the easel on which the print is to be made. The uniform vacuum of the easel is also problematic when used to hold a sheet of material that does not cover the entire easel. This is because the force exerted on the smaller sheet of material is substantially lowered due to it the lack of resistance in pulling air through parts of the easel that are not covered by the material. As a result, the easel cannot hold the sheet of material with enough strength as may be desired.
An additional problem with conventional easels is the inability to place successive sheets of material in the exact same location on the easel. For example, with photographic film printing, multiple prints cannot be made from the same negative without readjusting the equipment if subsequent sheets of photographic paper cannot be placed exactly where the first one was placed when the original adjustments were made.
Other problems with conventional easels include the tendency of some sheets of material to "curl up" when placed on the surface of the easel and the tendency of the easel to become warped. Such curling of material or warpage of the easel results in a print made on the easel being distorted. To prevent such distortion, Applicant has, more than one year prior to the filing date of the present application, but after the filing date of the parent application, sold a vacuum easel including a corrugated panel sandwiched between stiff supporting structures. Corrugated plastic panels and boards have been satisfactorily used for a wide variety of products as represented by U.S. Pat. No. 4,760,928, entitled "Corrugated Sheet Plastic Beverage Display Case;" U.S. Pat. No. 4,803,813 entitled, "Foldable Corrugated Plastic Roof Ventilator," and U.S. Pat. No. 4,828,894, entitled "Corrugated Plastic Board Assemblies." Also, various processes are well known to form a wide variety of articles from corrugated plastic material. U.S. Pat. No. 4,428,651, entitled "Thermal Forming Process of Corrugated Plastic Board" discloses one example of these processes. U.S. Pat. No. 5,304,056, entitled "Apparatus for Sealing Edges of Corrugated Plastic Material" provides further examples of fabricating techniques using corrugated plastic material. Each of the preceding patents is incorporated by reference for all purposes within this application.
Although the vacuum easel with supporting structures reduces warping and thicknesses associated with conventional easels, it did not provide sufficient force to hold smaller sheets of material, permit successive sheets of material to be placed in the same location as the original or prevent air flow around edges of the sheet of material to allow the sheet to remain flat on the easel surface.